Examples of a method for forming an aluminum film on a base include (i) physical vapor deposition (PVD) methods, such as vacuum deposition methods, sputtering methods, and laser ablation methods; (ii) paste coating methods, and (iii) plating methods.
(i) PVD Method
In a vacuum deposition method, for example, raw material aluminum is melted and evaporated by irradiating an aluminum alloy with an electron beam to deposit aluminum on a resin surface of a resin body having communicating pores, thereby forming an aluminum layer. In a sputtering method, for example, an aluminum target is subjected to plasma exposure to evaporate aluminum. The evaporated aluminum is deposited on a resin surface of a resin body having communicating pores, thereby forming an aluminum layer. In a laser ablation method, for example, an aluminum alloy is melted and evaporated by laser irradiation to deposit the aluminum alloy on a resin surface of a resin body having communicating pores, thereby forming an aluminum layer.
(ii) Paste Coating Method
In a paste coating method, for example, an aluminum paste in which an aluminum powder, a binding agent (binder), and an organic solvent are mixed together is used. The aluminum paste is applied onto a resin surface and then heated to eliminate the binder and the organic solvent simultaneously with the sintering of the aluminum paste. This sintering may be performed at a single operation or a plurality of operations. Alternatively, for example, the aluminum paste may be sintered simultaneously with the thermal decomposition of a resin body by applying the aluminum paste, heating the aluminum paste at a low temperature, and heating the aluminum paste with the aluminum paste immersed in a molten salt. Furthermore, this sintering is preferably performed in a non-oxidizing atmosphere.
(iii) Plating Method
Plating aluminum in an aqueous solution is practically almost impossible. Thus, an aluminum layer may be formed on a resin surface of a resin body having communicating pores by a molten salt electrolytic plating method in which aluminum is plated in a molten salt. In this case, aluminum is preferably plated in a molten salt after a resin surface of electrical conduction treatment in advance.
As the molten salt used for the molten salt electrolytic plating, a salt, for example, lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), or aluminum chloride (AlCl3), may be used. Furthermore, salts containing two or more components may be mixed to form a eutectic molten salt. The eutectic molten salt advantageously has a low melting temperature. The molten salt is required to contain aluminum ions.
In the molten salt electrolytic plating, a multicomponent salt, such as AlCl3—XCl (X: alkali metal)-MClx (M is an additive element selected from Cr, Mn, and transition metal elements), is used. The salt is melted to prepare a plating liquid. A base is immersed therein and subjected to electrolytic plating, so that the base surface is plated with aluminum. In the case where a base is composed of a non-conductive material, a base surface is subjected to electrical conduction treatment as a pretreatment in advance. Examples of the electrical conduction treatment include the plating of a conductive metal, such as nickel, on a resin surface by electroless plating; the coating of a conductive metal, such as aluminum, on a base surface by a vacuum deposition method or a sputtering method; and the application of a conductive paint containing conductive particles, such as carbon.
Conventionally, aluminum foil has been produced by rolling an aluminum strip. PTL 1 describes a method for producing aluminum foil by rolling. Specifically, as illustrated in FIG. 3, strips A and B coiled around take-up and supply reels 101, 102, 103, and 104 are rolled in two or more passes with a reversible rolling mill 105 equipped with work rolls 106 and back-up rolls 107 by passing the strips A and B through a nip between work rolls 106, thereby producing sheets of aluminum foil.
Aluminum foil has properties, such as thermal conductivity, moisture resistance, impermeability to air, lightness, and light-shielding capability and thus is used as a packaging material for various articles. In addition, aluminum foil is commonly used as a material for positive-electrode collectors of electrolytic capacitors and lithium-ion batteries because of its excellent electrical conductivity.
For example, in the case where aluminum foil is used for positive-electrode collectors of lithium-ion batteries, aluminum foil is used in the form of, for example, a stack or coil of multiple sheets of aluminum foil in order to increase battery capacity. PTL 2 relates to a negative electrode for a lithium secondary battery and discloses that copper foil is subjected to electrolytic plating to form a protruding portion on a surface of the copper foil.
Regarding copper foil used as an electrode material similarly to aluminum foil, a method is performed in which a film of copper plating is formed on a base by an electrolytic plating method, and then the film of plating is peeled from the base to produce copper foil. For example, PTL 3 discloses a method for producing copper foil used for a printed circuit board by electrodepositing copper on a cathode drum rotating in an electrolytic cell to which a liquid electrolyte is fed and separating copper foil from the cathode drum with the cathode drum rotating.